zBoy
a multiplatform, full-featured GameBoy classic emulator
... of the lack of a battery-backed RAM on the cartridge, and improving screen's resolution output using graphic algorithms like Scale2x, Scale3x, eagle... Here you can see a comparison between graphic algorithms supported by zBoy. This is also one of the very few GameBoy emulators that provides a 2-gameboys link emulation, which makes it possible to play some games in 2-players mode on a LAN.
The emulator is available for Linux, DOS and Windows (but should compile for other systems, too).